In a consumption ultrasonic flow meter of the transit time type, the determination of the consumption is based on a flow rate measurement of the consumed medium. The flow rate measurement is based on measuring a transit time difference between counter-propagating ultrasonic waves in the medium flow. The ultrasonic waves are generated by piezo-electric ultrasonic transducers. The typical transit times of the ultrasonic signals through the medium are very short, and therefore the transit times, or at least the time differences, need to be measured with a very high accuracy. One important factor in relation to the accuracy of the measurement is the impedance of the electronic arrangement as seen by the transducers when they act as transmitter and as receiver of ultrasonic signals. Any difference in the impedances in these two situations will translate into a measurement uncertainty.
EP 0 846 936 discloses an ultrasonic flow meter. It discloses that the electronic arrangement for controlling the ultrasonic transducers comprises a separate generator circuit and a separate receiver circuit that are connected to each other by a direct electrical connection, where the ultrasonic transducers are connected to this direct connection via switches. In this setup, each transducer sees the same low output impedance of the generator circuit and the same high input impedance of the receiver circuit, irrespectively of whether it works as a transmitter or as a receiver.
DE 100 48 959 C2 discloses an ultrasonic flow meter with an electronic arrangement for controlling the ultrasonic transducers where the generator circuit and the receiver circuit are combined in a single circuit, and each transducer sees the same impedance as defined by an input/output resistor irrespectively of whether it works as a transmitter or as a receiver.
FR 1.355.584 discloses an ultrasonic flow meter with an electronic arrangement for controlling the ultrasonic transducers, where the electronic arrangement ensures that the impedances in front of the transducers are extremely small both in transmission and reception modes, and each transducer consequently sees a very small impedance irrespectively of whether or not it works as transmitter or receiver, and thereby any differences between the impedances are also very small.
In these systems, switches are shifted between the transmitting and receiving transducers while the ultrasonic signal propagates through the medium. While the switching itself may be practically instantaneous, any electronic transients arising from the switching need to die out before an accurate measurement can be made. Consequently a certain distance must be covered by the ultrasonic wave inside the flow meter, hindering among other things, the fabrication of small flow meters.
WO 94/17371 discloses an ultrasonic flow meter with an electronic arrangement, where switches are applied in a manner so that there is not a need for switching while the ultrasonic signal propagates through the medium, and consequently the length of the flow tube is not limited by the time it takes switch related electronic transients to die out. However, in the disclosure of WO 94/17371, the electronic arrangement requires that each flow meter circuit needs to be impedance matched for so-called reciprocal operation. This is a time consuming and expensive operation to perform on each flow meter circuit.